1. Field of the Invention
This invention relates to wire and cable. In one aspect the invention relates to tree-resistant, crosslinkable polyethylene compositions while in another aspect, the invention relates to insulation sheaths and other cable coverings comprising such compositions. In still another aspect the invention relates to such compositions and cable coverings comprising a thiobis phenolic antioxidant while in yet another aspect, the invention relates to a process for mixing the thiobis phenolic antioxidant with the polyethylene.
2. Description of the Related Art
U.S. Pat. No. 6,869,995 describes a composition comprising polyethylene (PE), a thiobis phenolic antioxidant and a polyethylene glycol (PEG) with a molecular weight in the range of 1000 to 100,000. This composition is useful for the manufacture of electric power cable insulation sheathing with good scorch resistance during manufacture and good water-tree resistance during use. The PE is compounded with PEG in a two-roll mill operating at 24 revolutions per minute (rpm) on the front roll and 36 rpm on the back roll and a temperature of 125 to 130° C. on the two rolls for 10 minutes. In one embodiment, the procedure involves preheating the resin to 70° C. in an oven; fluxing the resin as quickly as possible on the two-roll mill (3 to 4 minutes); adding PEG and 4,4′-thiobis-(2-tert-butyl-5-methyl-phenol) (TBM6) and fluxing for an additional 3 to 4 minutes; and then adding peroxide and fluxing, peeling, and folding until well mixed. The patent does not discuss the use of a pre-formed blend of TBM6 and PEG.
TBM6 is a high performance thiobis phenolic antioxidant and scorch retardant used in crosslinkable (XL) PE compositions, and it has a high melting temperature of 162° C. The high melting point of this additive causes several problems in the production of insulation compounds for wire and cable applications. These problems include:
(A) TBM6 is not highly soluble in polyethylene resins which can result in unmelted TBM6 particles in the polyethylene insulation compositions. These unmelted particles of TBM6 are undesirable in XLPE because they can cause unacceptable electrical properties in finished cables produced from the material.
(B) The high melting point of TBM6 creates difficulty for using this additive in liquid additive feeder systems at the compounding facility and thus makes the accurate metering of the additive into the compound difficult. For those compounding facilities that employ only liquid additive feeding systems, TBM6 is simply not an option for use in the compound formulation.
(C) The high melting point of TBM6 creates difficulty in filtering the additive at the compounding facility to improve the additive cleanliness. Cleanliness is an important feature of XLPE insulation compositions due to the likelihood for impurities and defects to cause electrical failures in finished cables produced from XLPE. Improved TBM6 additive cleanliness translates to improved finished XLPE insulation compound cleanliness.
(D) The powder form of TBM6 is considered an explosive dust. This necessitates the use of costly explosion-proof materials handling equipment in order to use the powder form of TBM6. Otherwise, the more costly compacted pellet form of TBM6 must be used to minimize dust accumulation (TBM6 has a low minimum ignition energy (MIF) and a high dust deflagration index (Kst)).
PEG 20000, which is used as a tree retardant additive in crosslinkable polyethylene (XLPE) insulation compositions for wire and cable applications (U.S. Pat. Nos. 4,305,849, 4,440,671, and 6,869,955), has a melting point of about 62° C.